For viewing of a virtual image in a magnified form by a viewer, a virtual image viewing optical system as shown in FIG. 1 has been proposed.
In the virtual image viewing optical system shown in FIG. 1, an image light displayed on an image display device 301 is first incident upon an optical waveguide 302 having a transmission hologram lens 303 provided therein. While being formed by the transmission hologram lens 303 into a parallel light, the incident image light is deflected at an angle for total reflection inside the optical waveguide 302.
The optical waveguide 302 has also a transmission hologram grating 304 provided therein in line with the transmission hologram lens 303 at a predetermined distance from the latter. After traveling through the optical waveguide 302 while being totally reflected, the image light is incident upon the transmission hologram grating 304 by which the image light is diffracted again and projected in the parallel-light state to outside the optical waveguide 302 and toward the viewer's pupil.
For viewing of a virtual image in a magnified form by a viewer, there has also been proposed a virtual image viewing optical system as shown in FIG. 2.
In the virtual image viewing optical system shown in FIG. 2, an image light displayed on an image display element 401 is directed for incidence upon an optical waveguide 403 through a free-form surfaced prism 402. As shown in FIG. 3, the optical waveguide 403 includes a first HOE (Holographic Optical Element) 404 and second HOE 405 provided in an incident region Z1 at the incident side of the optical waveguide 403, and a third HOE 405 and fourth HOE 406 provided in an outgoing region Z2 at the outgoing side. The image light incident upon the optical waveguide 403 is continuously diffracted and reflected at the light-incident side of the optical waveguide 403, first HOE 404 provided on a surface opposite to the light-incident side and at the second HOE 405 provided at the light-incident side, and deflected inside the optical waveguide 304 to go at a larger angle than a critical angle for total reflection. More specifically, image light L1 incident upon the optical waveguide 403 is diffracted and reflected at a first incident-side diffraction-reflecting surface D1 of the first HOE 404 and then at a second incident-side diffraction-reflecting surface D2 of the second HOE 405 to go at a larger angle α2 than the critical angle. It should be noted that when the image light L1 is diffracted and reflected at the first diffraction-reflecting surface D1, it will go at a smaller angle α1 than the critical angle.
The image light L2 directed at the larger angle than the critical angle inside the optical waveguide 403 travels while being totally reflected inside the optical waveguide 403, and is then continuously diffracted and reflected at a first outgoing-side diffraction-reflecting surface D3 of a fourth HOE 407 and then at a second outgoing-side diffraction-reflecting surface D4 of a third HOE 406 to go at a smaller angle α3 than the critical angle and outgo toward the optical pupil of the viewer outside the optical waveguide 403.
However, the virtual image viewing optical system shown in FIG. 1 is disadvantageous as will be described below:
Firstly, in the virtual image viewing optical system shown in FIG. 1, divergent light projected from an image display device 301 is incident directly upon the transmission hologram lens 303 in the optical waveguide 302. When the distance between the image display device 301 and transmission hologram lens 303 is increased, namely, when the focal distance of the transmission hologram lens 303 is increased, for an increased magnification of the optical system, the diameter of the pupil 305 cannot be increased because the transmission hologram lens 303 has only a relatively small diffraction acceptance angle.
Secondly, since the interference fringe of the transmission hologram lens 303 has a complicated structure having a spherical phase component, it is difficult to combine or laminate the interference fringes together for a larger diffraction acceptance angle and the lens 303 cannot be designed to diffract light rays equal in wavelength and incident angle to each other with different efficiencies at the same diffraction angle.
Thirdly, in the virtual image viewing optical system shown in FIG. 1, since the transmission hologram lens 303 provided on the optical waveguide 302 diffracts image light rays coming from the image display device 301 while forming the light rays into a parallel pencil of rays, that is, while generating an optical power, a large monochromatic eccentric aberration will be caused, which will also lead to a reduced resolution of an image projected on the pupil.
Fourthly, the virtual image viewing optical system shown in FIG. 1 uses the transmission hologram grating 304 to correct achromatic aberration occurring in the transmission hologram lens 303. Since light rays incident upon the transmission hologram grating 304 is deflected only in the direction in the plane of the drawing in FIG. 1, aberration occurring in a direction perpendicular to at least the drawing plane cannot be canceled. The diffraction-caused chromatic aberration takes place because the two transmission holograms (transmission hologram lens 303 and transmission hologram grating 304) provided in the optical waveguide 302 are different from each other and there can be used substantially only a light source of which the waveband is narrow, which is a large constraint to this conventional virtual image viewing optical system.
A simulation was actually made by retracing the light incident upon the pupil in the virtual image viewing optical system shown in FIG. 1. The result of simulation shows that even when the chromatic aberration was corrected by the two transmission holograms, it was found that a wavelength shift of ±2 nm resulted in a shift of ±30 μm of image light on the image display device 301.
If the two transmission holograms are identical transmission volume hologram gratings having no optical power, for example, another problem described below will take place.
It is well known that at a constant incident angle, the diffraction acceptance waveband of the transmission volume hologram is broader than that of the reflection volume hologram. Therefore, incase the waveband of a light source is broad or in case the wavelength interval of a light source for each of RGB (R: Red light; G: Green light; B: Blue light) that are three primary colors of light is narrow (in case the waveband of each color light is broad), chromatic dispersion due to vast diffraction, that is, diffraction chromatic dispersion will take place.
Even a transmission volume hologram prepared for green (of 550 nm in central wavelength), for example, has a diffraction efficiency of about 10% with a waveband of 400 to 630 nm and will partially diffract light from a blue LED (Light Emitting Diode) (of 410 to 490 nm in light-emitting wavelength) and light from a red LED (of 600 to 660 nm in light-emitting wavelength).
The chromatic aberration due to the diffraction chromatic dispersion can be canceled by two holograms equal in grating pitch to each other. However, in case the chromatic dispersion made by one of the holograms is larger, a pencil of rays traveling inside an optical waveguide will spread largely, resulting in a following problem. When the largely spread pencil of rays having been diffracted by the first hologram and traveled inside the optical waveguide is diffracted at the second hologram and projected from the optical waveguide, it will spread largely in the traveling direction on the basis of its wavelength and lead to a deteriorated color uniformity of a virtual image on the viewer's pupil.
On the other hand, in the reflection volume hologram, the diffraction acceptance waveband one interference fringe has is narrow. Therefore, in case image light is colored, the colors (total reflection angle inside the optical waveguide) can be equalized in diffraction angle by laminating hologram layers together for each of RGB or combining the interference fringe of each of RGB.
On the contrary, with a constant incident wavelength, the diffraction acceptance angle of the transmission volume hologram is smaller than that of the reflection volume hologram and thus it will be difficult to increase the diameter of the pupil 305 or field angle.
Also, since in the virtual image viewing optical system shown in FIGS. 2 and 3, an image of the image display element 401 is intermediate-formed inside the optical waveguide 403, the first HOE 404, second HOE 405, third HOE 406 and fourth HOE 407 should have an optical power in an eccentric layout. Therefore, also in this virtual image viewing optical system, eccentric aberration will occur as in the virtual image viewing optical system shown in FIG. 1.
In the virtual image viewing optical system shown in FIGS. 2 and 3, the free-form surfaced prism 402, first HOE 404, second HOE 405, third HOE 406 and fourth HOE 407 are provided axial-symmetrically with respect to each other to reduce the eccentric aberration. However, since the upper limit of the diffraction efficiency of each HOE is substantially 70 to 80%, the total of the diffraction efficiency of the four HOEs is the fourth power of 70 to 80% and thus the diffraction efficiency will be considerably lower.
As above, in a hologram having a complicated interference patter, it is difficult to increase the diffraction acceptance of the interference fringe by laminating hologram layers together or combining the interference fringe. Therefore, the pupil diameter cannot be increased.
Also, since convergent light (down to intermediate image formation) or divergent light (after the intermediate image formation) travels inside the optical waveguide 403, a pencil of rays not diffracted by the first reflection and diffraction but totally reflected again in the plane of the optical waveguide 403 cannot be used a any image display light or image light any longer. Therefore, the conventional virtual image viewing optical system can neither use light with any improved efficiency nor enlarge the viewable range.